1. Field of the Invention
The present invention relates to a copier/printer capable of reproducing duplex images of a document image, and, more particularly, to a duplex feeder of such a copier/printer which stacks sheets which have images on one side thereof and which feeds them back for imaging on the other side thereof.
2. Background of the Invention
When two pages of an original document are copied onto opposite sides of the same copy sheet, it is known as duplex copying. This procedure also has well known applications in printers where it is known as duplex printing. Since the apparatuses can be identical for duplexing in a copier and duplexing in a printer, this discussion will focus on duplex copying. Duplex copying requires the use of a dedicated duplex tray for storing a stack of copy sheets having a simplex image formed on one side thereof from a stack of either simplexed or duplexed original documents. The stack of copy sheets is stored in the duplex tray until the stack of original documents has been copied on a first pass and readied for copying onto the other side of the copy sheets on a second pass. The stack of copy sheets in the dedicated duplex tray is sequentially refed by feeder means to the copying station so that the images of the original documents can be copied onto the opposite side of the copy sheets to form duplex copies. The feeder means of the duplex feeder can be positioned on either the top or the bottom of the stack of copy sheets in the duplex tray. However, it is more desirable to use a bottom feeder means so that the first copy sheet deposited in the duplex tray will be the first sheet fed out of the tray. The term "duplex feeder" is meant to include both the duplex storage tray and the feeder means. Printers require a dedicated duplex tray if a burst mode of operation is used instead of an interleaving mode for productivity reasons.
Typical duplex feeder arrangements, such as the one disclosed in U.S. Pat. No. 4,116,558 to Adamek, include a single duplex tray with the feed means positioned so as to feed copy sheets from the bottom of the stack. The duplex tray arrangement disclosed in U.S. Pat. No. 4,538,906 to Brown also includes a single duplex tray however it shares operation with the feeder of an input copy sheet tray. U.S. Pat. No. 4,772,916 to Tani discloses an intermediate sheet feeder for temporarily stacking copy sheets. The sheet feeder includes first and second intermediate trays (only one of which is used for duplex copying), and first and second feeder devices associated with each tray.
However, arrangements such as these are subject to inherent stack weight limits associated with bottom feeders. Since the stack weight limit of bottom feeders corresponds to a maximum number of copy sheets that can be passed through the simplex run at any one time, the user is required to execute more copy runs if the number of documents to be copied exceeds the copy sheet storage capacity of the duplex feeder. If the storage capacity of the bottom feeder is exceeded, it often leads to mis-feeds and multi-feeds as the bottom feeder cannot properly support the weight of the copy sheets. To alleviate the stack weight problem it has been proposed to use a duplex feeder having two duplex trays and two respective duplex feeders. However, the additional hardware utilized in this approach is both costly and space consuming.
It is apparent from the foregoing that there is a need for a higher capacity duplex feeder. It is therefore a feature and advantage of the present invention to provide a duplex feeder which overcomes the maximum duplex copy sheet handling problem caused by the inherent stack weight limits of bottom feeders.
It is a further feature and advantage of the present invention to provide a duplex feeder which overcomes the above-mentioned problem at a lower cost than would be required to use two duplex feeders and trays.
It is a still further feature and advantage of this invention to provide a buffer tray in addition to the feeder tray for handling copy sheets which exceed a predetermined maximum due to the stack weight limit of the bottom feeder.
It is another feature and advantage of this invention to provide a cost effective and simple way of transferring a stack of simplex copy sheets from the buffer tray to the feeder tray.
It is yet another feature and advantage of the present invention to provide a buffer tray system that is applicable for any bottom feeders where high capacity is desired, e.g., recirculating document handlers and where the sequence of feeding is N-1.